1. Technical Field
The present disclosure is directed to electrosurgical systems, and, in particular, to a circuit and method for achieving gain compensation across varying operating conditions in an electrosurgical generator utilizing a full bridge topology.
2. Description of the Related Art
An electrosurgical generator is used in surgical procedures to deliver electrical energy to the tissue of a patient. When an electrode is connected to the generator, the electrode can be used for cutting, coagulating or sealing the tissue of a patient with high frequency electrical energy. During normal operation, alternating electrical current from the generator flows between an active electrode and a return electrode by passing through the tissue and bodily fluids of a patient.
The electrical energy usually has its waveform shaped to enhance its ability to cut, coagulate or seal tissue. Different waveforms correspond to different modes of operation of the generator, and each mode gives the surgeon various operating advantages. Modes may include cut, coagulate, a blend thereof, desiccate, seal, or spray. A surgeon can easily select and change the different modes of operation as the surgical procedure progresses.
In each mode of operation, it is important to regulate the electrosurgical energy delivered to the patient to achieve the desired surgical effect. Applying more than the correct dosage may result in tissue destruction, and may prolong healing. Applying less than the desired dosage of energy power inhibits the surgical procedure. Thus, it is desirable to control the output energy from the electrosurgical generator for the type of tissue being treated.
Different types of tissues will be encountered as the surgical procedure progresses and each unique tissue requires more or less power as a function of frequently changing tissue impedance. As different types of tissue and bodily fluids are encountered, the impedance changes and the response time of the electrosurgical control of output power must be rapid enough to seamlessly permit the surgeon to treat the tissue. Moreover, the same tissue type can be desiccated during electrosurgical treatment and thus its impedance will change dramatically in the space of a very brief time. The electrosurgical output power control has to respond to such impedance changes as well.
Three standard modes of control are commonly used during electrosurgical generation. At low tissue impedances, the generator controls to a current limit. At mid-range tissue impedances, the generator controls to a power limit. At highest tissue impedances, the generator controls to a voltage limit. Generally, the voltage, current, and power limits describe the electrosurgical mode. The generator must employ a stable control loop over the full impedance range whether controlling to voltage, current, or power.
In prior-art electrosurgical generator designs, voltage from the AC mains is rectified to provide a DC voltage. An inverter stage converts the DC voltage back to AC voltages at frequencies appropriate for the desired tissue effect. The output of this stage is an AC waveform that can be controlled to voltage, current, or power, to deliver the correct energy to tissue.
A common technique for configuring a variable DC power supply utilizes Phase Shifted Full Bridge topology wherein output power is controlled via changes in the duty cycle of a pulse-width modulated input signal. At any single operating point, the gain of a phase shifted full-wave bridge inverter is linear. However, the operating points may vary over a wide range due to a setpoint change, a load change, an impedance change, and changes in other parameters. Consequently, the overall gain of the inverter stage can vary significantly. This can have an impact on the controlled delivery of energy to tissue.